An investigation by electrical methods of lecithin-in-hydrocarbon films in aqueous solutions

Abstract

Black hydrocarbon films supported by a Teflon vessel have been formed beneath aqueous solutions of sodium, potassium and calcium chlorides. The films were stabilized by carefully purified egg lecithin. The d.c. conductances of the system of film and supporting vessel were irreproducible and generally < 10^-8 mho/cm² of film. A film, together with its adjacent aqueous phases, showed a dielectric dispersion characterized by one relaxation time. The films themselves had capacitances which were accurately reproducible and which were independent of frequency and of the nature and concentration of the electrolyte. The magnitude of the capacitances was 0.38±0.01 μF/cm². It was deduced that only the hydrocarbon part of the films contributed to this capacity. The composition of the hydrocarbon part of the films has been inferred from considerations of the interfacial tensions between the lecithin-in-hydrocarbon solutions and the aqueous solutions. From a knowledge of the bulk dielectric constant of the hydrocarbon mixtures, the thickness of the films was deduced from the parallel plate condenser formula. The thickness of the hydrocarbon part of the films was found to be 48±1Å. This value may be compared with twice the average chain length of the lecithin stabilizing molecules, which, on the basis of 18 carbon atoms per chain, is approximately 46Å.